1. Field of the Invention
The present invention relates, according to a first aspect, to the domain of synchronised control methods of a plurality of signal formatting equipment. According to a second aspect, the invention relates to a pair of signal formatting equipment, said equipment is used as backup to supply an item of transmission equipment and to reduce the impact on the transmission of a switch from one item of equipment to the other.
The present inventions relate more precisely to the field of networks of the type “Single Frequency Network” or “SFN”.
2. Description of the Prior Art
In the prior art, an SFN network is a network of transmitters, radio or analogue or digital television, operating over a single frequency in a determined region. As shown in FIG. 1, all the transmitters constituting this network, whether they are adjacent or not, use an identical frequency F1 to send an identical signal. In FIG. 1, a transmitter is constituted by a modulator 10, 20, 30 and by an antenna 15, 25, 35. The advantage of such an architecture is double: it enables, on the one hand, to limit the frequencies used over a territory. Indeed, in a non-SFN transmitter network, if a frequency is used by a first transmitter, this frequency cannot be used by the transmitters adjacent to this first transmitter otherwise interference phenomena would be created. Moreover, it can improve the reception quality since, in an SFN network, the signals received from several adjacent transmitters are no longer destructive between each other but on the contrary constructive.
To implement such a network architecture, it is necessary to comply with certain conditions: the transmitters constituting the network must all use the same modes of modulation and they must also transmit exactly the same signal temporally. For this DVB standard specifies, in the document “ETSI TS 101 191, V1.4.1 (2004-06) Digital Video Broadcasting (DVB); DVB mega-frame for Single Frequency Network (SFN) synchronization”, a mechanism making it possible to synchronize the streams emitted by different transmitters of an SFN network.
The signal to emit is broken down into megaframes whose length depends on the modulation mode chosen for the transmission. Synchronisation packets, frequently designated using the name “Megaframe Initialisation Packet” or using their acronym “MIP”, are inserted into the signal to send. They contain temporal pointers that enable the transmitters to position these frames exactly in time on the basis of an extremely accurate time base present at the level of the transmitters and which is common to them. The time base is for example of the GPS type. Hence, when the time base has the form of a signal (pulse) at 1 Hz and a clock signal at 10 MHz, these two frequencies being perfectly stable, the MIP points to the start of the next megaframe that starts for example on the 1120th clock pulse following the last pulse to date delivered by the time base.
The signal to transmit TS_SFN1, that will be assumed to be SFN formatted, is generated by an item of formatting equipment FE1 from a signal TS. In particular, the formatting equipment FE1 inserts the MIP synchronisation packets, not shown in FIG. 1, in the TS signal. The signal TS_SFN1 is sent to several modulators 10, 20, 30 that all generate a modulated signal strictly identical and in phase for each one of its associated antenna 15, 25, 35.
As for any critical link in a transmission system, it is strongly advisable to be able to have a redundant architecture for the formatting equipment FE1. It is thus hoped to overcome one of the faults of the item of equipment FE1 or to authorise maintenance actions without interrupting the service. Most often, recourse is made to a standard architecture, shown in FIG. 2 that consists in supplying a modulator MOD, 10 by at least two items of formatting equipment FE1, 1; FE2, 2 in a parallel configuration. Each item of equipment EF1, EF2 thus receives the same signal TS and produces a formatted signal SFN: TS_SFN1, TS_SFN2 intended for the modulator MOD.
To make the explanations clearer, a switch SW, 9 receives the signals TS_SFN1 and TS_SFN2 and sends either TS_SFN1 or TS_SFN2 to the modulator MOD according to whether one of the two items of equipment EF1, EF2 is faulty or out of service during a maintenance operation. The switch SW can naturally be inserted into the modulator MOD that will then be provided with 2 inputs.
As things currently stand, this architecture has the disadvantage of not allowing one formatted signal SFN to be switched to the other without having a noticeable effect on the transmission. Indeed, the structure of the megaframes is imposed by the modulation mode and is consequently recognised by the two items of equipment FE1, FE2 that thus generate identical megaframes.
In the rest of this document, it has been chosen to represent the formatted streams as a succession of megaframes MF1, . . . , MFi, MFi+1, MFn where i is an index identifying each megaframe in a unique manner. The megaframes MF1, . . . , MFn all have a known identical duration TMF. The duration of the megaframes is identical and is noted as TMF.
The date of the start of transmission of the first megaframe is left to the free choice of each item of equipment FE1, FE2: it is basically related to a choice made when the items of equipment FE1, FE2 are powered up.
Hence, considering a handover of the signal TS_SFN1 to the signal TS_SFN2 at a date TSW as shown in FIG. 3, the resulting signal TS_SFN3 contains, at the moment of the handover, a megaframe (here MFn−2) of an abnormally large size (or small depending on the case). In a transitory manner, the modulator, MOD in the presence of such a signal can no longer generate a coherent modulated signal. It must wait to find an input signal compliant with the type of modulation that is its own to generate a modulated signal again. At worst, two successive megaframes are lost on each handover.
For the handover not to lose any megaframes on transmission, it therefore appears advisable to ensure that the two items of equipment FE1, FE2 redundantly supply the modulator generating megaframes (MF1, . . . , MFn) perfectly in phase, with an accuracy in line with the required accuracy for implementing an SFN network.
The most immediate solution to the problem posed by a generation of signals in phase by a plurality of backup equipment FE1, FE2 consists in interconnecting the equipment EF1, EF2 together by defining an information exchange protocol between the equipment FE1, FE2 to ensure that this backup equipment generates signals TS_SF1, TS_SFN2 in phase. But this solution is a source of strong architectural and interconnection constraints. In addition, it is poorly adapted to an ‘n+p’ architecture where ‘n’ items of formatting equipment (not necessarily generating the same megaframe structures) are backed up by ‘p’ items of formatting equipment. Moreover, this architecture is extremely vulnerable to the network latency that can lead the equipment FE1, FE2 to take non-phased decisions.
One of the purposes of the present invention is to overcome these different disadvantages.